Perhaps a rogue Transformer, not directly affiliated with the Decepticons or Autobots. This could be a Transformer who has his own agenda. He might think that Miko holds the key to something that can change the balance of power in the Transformers' universe. So, he kidnaps her to further his own goals, regardless of the consequences for the Autobots or Decepticons.
A Decepticon could be the kidnapper. Decepticons are always looking for ways to gain an advantage over the Autobots. They might see Miko as an easy target to get to the Autobots. For instance, Starscream, being power - hungry and sly, could potentially kidnap Miko to show his worth to Megatron or to carry out his own devious plans.
A likely kidnapper could be Starscream. He's always looking for ways to gain an advantage over the Autobots and Arcee would be a valuable hostage. He might think that by kidnapping her, he can force Optimus Prime to make concessions or reveal secrets.
One possible motive could be to use Miko as a bargaining chip. Maybe the kidnappers want to exchange her for some valuable object or information within the Transformers' world. For example, they might be after a powerful Energon source or a secret Autobot technology.
Jack and Miko are popular in fanfiction because they are relatable human characters in a world full of giant robots. Their interactions with the Transformers add a unique dynamic to the stories.
It could be an old enemy of Keith's, someone from his past who has a vendetta against him. Maybe a former rival from his days in the military academy who has now turned to more devious means.
In some creative fanfiction, it could be an alternate version of Spiderman himself from a different dimension. This Spiderman has gone bad and kidnaps a fan from our dimension as part of some grand, dark plan that involves crossing dimensions and causing chaos.
A power - hungry nobleman from a neighboring land could be the kidnapper. He might believe that by having Rapunzel in his possession, he can gain some sort of political advantage or use her as a bargaining chip against her kingdom.
It could be Orochimaru. He's always been interested in obtaining powerful individuals for his experiments, and Naruko with her unique abilities would be a prime target.
It could be a new villain that has emerged, someone who is jealous of Gohan's power and wants to make it his own. This villain might be from a far - off planet and has heard of the Saiyan legend.
It could be a group of rogue Decepticons who are acting on their own. They might not follow Megatron's orders but still see value in kidnapping Arcee. Maybe they have a personal vendetta against her or think they can get rich by selling her to the highest bidder in the Cybertronian underworld.
Maybe it's a group of villains working together. They could be from different factions but united by the idea that Izuku has the potential to become a huge threat to their plans. They might kidnap him to either eliminate him or turn him to their side. Also, it could be a lone, obsessed fan of a villain who wants to prove themselves by bringing Izuku to their idol.